Various methods, “modalities”, for imaging internal features of a region of interest (ROI) of a person's body for diagnostic purposes are known. Among the imaging modalities are by way of example, the familiar X-ray and ultrasound (US) imaging modalities, computerized tomography (CT), magnetic resonance imaging (MRI), and the nuclear medicine imaging techniques referred to as positron emission tomography (PET) and single photon emission computerized tomography (SPECT).
All of the various modalities employ sensors that receive and register amounts of radiation, hereinafter also referred to as “imaging radiation”, that is reflected or emitted by the features, transmitted through the features, or emitted by substances located in the features. Imaging radiation may for example comprise, X-rays (CT imaging), γ-ray photons (PET and SPECT imaging), radio frequency (RF) electromagnetic waves (MRI) and ultrasound (US imaging). The amounts of imaging radiation from the ROI that are registered by the sensors and associated with each of a plurality of voxels in the ROI are used to provide an image of the patient's features in the ROI.
By way of example, PET scans of a ROI of a patient are produced by introducing a biologically active “carrier” molecule that is tagged with a positron emitting radionuclide into the patient's body. The molecule concentrates in various regions of the ROI depending on features in the ROI and a type of biological activity that characterizes the carrier molecule. Positrons emitted by the radionuclide in voxels of the ROI at which the molecule concentrates annihilate with electrons in the voxels and produce pairs of “back-to-back” photons that propagate out of the voxels and the patient's body along opposite, collinear directions. A PET scanner comprising sensors that detect pairs of back-to-back photons leaving the patient's body determines from which voxels in the body the back-to-back photons originate to map the concentration of the molecule in the body. The concentration map shows which features in the ROI preferentially accumulate the molecule and may be used to image the features, characterize their morphology and/or metabolic functioning. PET imaging is often used to locate and image cancerous growths in a patient's body.
The various medical imaging modalities are subject in varying degree to motion blurring, which degrades sharpness of images the modalities provide. The longer an exposure period a given medical imaging modality requires to sense and register a sufficient amount of imaging radiation to acquire a satisfactory image of a patient, the more sensitive the modality is to blurring resulting from motion of the patient during the exposure period. Besides “fidget” motion of a patient during an exposure period, which may be subject to a satisfactory degree of control, relatively difficult or impossible to control motion of body organs that accompany the respiratory and cardiac cycles of the patient contribute to motion blurring. In particular, PET or SPECT, which require relatively long exposure periods because the flux of imaging radiation (γ-ray photons) that they image is typically relatively weak, are sensitive to motion blurring.
Various windowing techniques have been developed to compensate a medical imaging modality for motion blurring in an image it acquires of a patient that is caused by motion accompanying the patient's cardiac or respiratory cycles. Generally, the windowing techniques divide an exposure period during which the modality registers imaging radiation into a plurality of relatively short duration “imaging windows”, for each of which an amount of the imaging radiation is measured. The imaging windows are configured so that during the exposure period there are a same whole number “N” of imaging windows for each of the patient's cardiac or respiratory cycle. The imaging windows are phase synchronized to the cycles so that every N-th window in the plurality of windows corresponds to substantially a same phase of the cycles. Configuring the imaging windows so that there are N imaging windows per cycle and that the imaging windows are phase synchronized is typically done by monitoring the cardiac or respiratory cycles with a motion sensor to sense phases of the cycles and when the cycles begin and end. Measurements of imaging radiation are labeled with cycle phases that are simultaneous with times at which the measurements are made and the phase labeled measurements are processed responsive to their respective associated phase labels to bin the measurements in phase synchronized windows.
Amounts of imaging radiation registered during imaging windows corresponding to a same given phase of the cardiac or respiratory cycle may be added, and the summed amount of imaging radiation is used to provide an image, hereinafter also referred to as a “phase image” of the patient's features for the given phase of the cycle. A phase ordered sequence of phase images, acquired for a ROI of a patient for different phases of the cycle may be used to provide a motion picture of the patient's features in the ROI that show how the features move during the cardiac or respiratory cycle.
Phase images of the features of an ROI are expected to have improved sharpness because feature displacement caused by cardiac or respiratory motion during the imaging windows is limited due to the relatively short duration of the windows. It is noted however, that whereas duration of the imaging windows, and as a result motion blur in phase images, decreases with increase in N, statistical, “shot noise”, increases with increase in N. If N is too small, shot noise may offset gains in image blur and degrade phase images to a degree at which the image is no longer satisfactory.